The Kingdom of Poland hails from a long line of nearly uninterupted Polish states. The Polish state's existance has only been interupted once in nearly a thousand years, and this for only a brief period of time. The Polish have at times owned nearly the entirety of Eastern Europe, going so far as to own the Third Rome, Moscow, for many years.
In recent time though the Polish have had a much tougher time with their neighbors as their system and government broke down from weak leaders and poor judgment and they were partitioned by Prussia, Austria, and Russia. Then Napoleon brought back the Poles from the dustbin of history as the Duchy of Warsaw, which they existed as for 87 years.
They were then granted the status of the Kingdom of Poland once more in 1894, and because of the personal union between Saxony and Poland it was proclaimed as the Polish-Saxon Commonwealth, it then existed in relative peace until 1896 when they invaded the Austro-Hungarian Monarchy to liberate Galician Poles from their oppresive masters.
Thus started the Great War of Liberation which was waged between the major powers of Europe. The war though soon ended and in 1897 the Austro-Hungarians ceded Galicia to the Poles in exchange for them to no longer be allies of the French. The Prussians signed a separate peace treaty with the Poles that ceded Saxony to the Prussians in exchange for a direct route to the sea in 1898.
The Poles then proclaimed the Kingdom of Poland with the lost of their Saxon holdings.